Fighters have been using a change in weight class as a way to re-surge their career since weight classes were implemented in MMA. It's an easy way to get another a second chance against new competition with the understanding that you will be more successful in the new weight class.
According to statistics though, a change in weight class is usually a short term way to prolong a career, not a way to find success. Out of 107 fighters that have made a permanent weight class change while in the UFC, only 38 ended up with a winning record in their new weight class. Meanwhile, 45 of the fighters ended up with losing records in their new weight class, along with 24 having an even record.
The most damning statistic for weight class changes comes when comparing the fighter's record in their new weight class to their old weight class. Only 20 out of 107 fighters ended up with a better record in their new weight class, 42 ended up with a comparable record while 45 ended up with a worse record.
Out of the 107 fighters that made permanent weight class changes, 90 fighters made the change to a lower weight class while 17 moved up in weight. The statistics also showed that moving up in weight and moving down in weight resulted in a similar winning percentage, 52.38% for up and 52.16% for down. The winning percentage of fighters before a weight class change was 57.1%.
Some fighters have found some form of success when changing weight classes as 10 of the 107 fighters ended up with title shots in their new weight classes, with two eventually winning a championship (Sean Sherk and Daniel Cormier).
In order to qualify as a change in weight class, the fighter must have fought twice in the UFC under a certain weight class before making the change to a new weight class. Fighters who bounced between two weight classes throughout their careers (Vitor Belfort, Randy Couture, B.J. Penn, Rich Franklin, etc.) were not included in the statistics. The purpose of the data is to find if permanent weight class changes are successful, rather than track fighters who fall in-between weight classes, like Franklinweight at 195lbs.
The reason for the decrease in success for fighters who change weight classes can be the result of multiple things. Fighters who go down in weight often give up the speed advantage that they had in their heavier weight class. Fighters also dehydrate themselves more to reach a lower weight class, meaning they have less energy and are more susceptible to a knockout.
There is also the reasoning that the lower winning percentage is just a result of a fighter getting older. Very few fighters end up with higher winning percentages the longer they continue fighting. The data is not meant to prove that weight class changes do not work at all, but that changing weight classes isn't a reliable way to make a career resurgence when struggling to stay in the UFC.
Diego Sanchez and Myles Jury will both be moving from the Lightweight division to the Featherweight division for their upcoming fights, so it will be interesting to see if either can find success with the change in weight class. Sanchez had a 2-2 record in his recent stint at lightweight while Myles Jury went 6-1.
The 107 documented UFC fighters had been fighting or had made their debut in the UFC between January 2005 and December 2013. The fight statistics are updated as of July 31st, 2015, with the last event recorded being UFC on Fox: Dillashaw vs. Barao 2.